NEMS switch with 30 nm-thick beam and 20 nm-thick air-gap for high density non-volatile memory applications
Introduction
Steady scaling of complementary metal–oxide–semiconductor (CMOS) devices has been a significant stimulus for a tremendous advancement in the semiconductor industry over the past four decades. However, many difficult challenges were encountered such as short channel effects, junction leakage and gate oxide leakage as the CMOS design rule is scaled into the nanometer regime [1], [2]. Recently, memory devices based on microelectromechanical systems (MEMS) [3], [4], [5] and nanoelectromechanical systems (NEMS) [6], [7], [8], [9] switches have been reported as one of the promising solutions because they show excellent on/off characteristics due to an essentially zero off current and practically infinite sub-threshold slope, and robustness under harsh environments such as radiation and low/high temperature.
MEMS have already had a noteworthy impact on RF circuit, the automobile, the aerospace and information technology areas [10]. NEMS are a thousand times smaller than MEMS and have the potential to enable revolutionary advances for future applications. Especially, extensive studies have been carried out on NEMS applications of suspended carbon nanotubes (CNTs) due to their superior electrical and mechanical properties [11], [12], [13], [14], [15], [16], [17], [18] but existing devices based on the CNTs grown using a “bottom-up” approach are still far from realization due to many obstacles such as difficulty in controlling the position and population of each CNTs.
In this paper, a NEMS cantilever switch (NCLS) and a NEMS clamp switch (NCS) with 30 nm-thick titanium nitride (TiN) beam and 20 nm-thick air-gap were fabricated by a conventional CMOS process. We successfully investigated the electrical characteristics of the two types of NEMS switches showing ideal on/off current property and repetitive operations under dc and ac bias conditions. Moreover, the suspended beam memory (SBM) cell structure based on the NCLS was proposed as one of the promising candidates for high density non-volatile memory applications.
Section snippets
Device structure and fabrication
Fig. 1a shows schematics of the two types of NEMS switches. Both the NCLS and the NCS consist of a word line (WL) electrode which is isolated from the adjacent ones by shallow trench isolation (STI) oxide, and a suspended beam hanging over the WL electrode which is anchored on the STI oxide. When applying a voltage between the suspended beam and the WL electrode, the induced electrostatic force pulls the suspended beam down towards the WL electrode, eventually brings it into contact with the WL
Results and discussion
Fig. 3a and b shows the I–V plots of the two types of fabricated NEMS switches. The NCLS (W/L/t = 200 nm/300 nm/30 nm) had a counter-clockwise hysteresis curve with the pull-in voltage of about 13 V and the pull-out voltage of about 8 V, as shown in Fig. 3a. In the case of the NCS (W/L/t = 200 nm/1000 nm/30 nm) as shown in Fig. 3b, the pull-in voltage was measured to be about 11 V but an abrupt pull-out characteristics was not clearly shown probably because the force that could restore the long NCS to its
Conclusion
The NCLS and the NCS of 30 nm-thick TiN beam and 20 nm-thick air-gap were successfully fabricated using conventional “top-down” CMOS fabrication. The fabricated NCLS demonstrate ideal on/off current characteristics with an essentially zero off current, a sub-threshold slope of less than 3 mV/decade and an excellent on/off current ratio over 105 in air ambient. Furthermore, the NCLS showed the repetitive switching operation of over several hundred of switching cycles under ac and dc bias condition
Acknowledgments
This work was supported in 2007 by Samsung Electronics Co., Ltd. and in part the Brain Korea 21 Project of the School of Information Technology at the Korea Advanced Institute of Science and Technology.
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